I the "Python Library Reference", about random module, I read :

betavariate(alpha, beta)
    Beta distribution. Conditions on the parameters are alpha > -1 and beta > -1. Returned values range between 0 and 1.

But

>>> import random
>>> random.betavariate(-0.5, -0.5)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/usr/lib/python2.4/random.py", line 575, in betavariate
    y = self.gammavariate(alpha, 1.)
  File "/usr/lib/python2.4/random.py", line 458, in gammavariate
    raise ValueError, 'gammavariate: alpha and beta must be > 0.0'
ValueError: gammavariate: alpha and beta must be > 0.0


The documentation is probably inaccurate, because they are two beta distribution usage in the math world:
one with alpha and beta >0 (the common one)
and the other whit alpha' and beta' > -1 where alpha'=alpha-1 and beta'=beta-1

this could explain the mistake.

Finally, I thing the documentation should specify :

alpha > 0 and beta > 0

The problem exist in Python 2.4.3 and probably in all versions.

This is already fixed in the development docs. Thanks for reporting it anyway!